GLACIATION OF THE YAKUTAT BAY REGION 205
One might expect important influence from uprising bottom currents, and, as is shown in a succeeding section, there is evidence that there is such influence in the production of interior flats. But, in general, there does not seem to be much effect from uprising of ice, for the moraines which cover the piedmont bulbs are prevailingly of angular material, such as falls upon glacier surfaces and in glacier reservoirs, not such as has undergone the scouring necessary in bottom ice layers. It is hoped that further study of Alaskan glaciers may yield facts which we lack at present concerning the nature of ice motion in piedmont bulb areas.
The piedmont bulb areas which we have so far studied have at some previous stage of greater expansion spread to their present extent, and doubtless farther. Whether this expansion was one of rapid or slow advance we do not know, nor can we assign to it an exact date. In the cases of some, however, notably the Lucia, Atrevida and Galiano Glaciers in Yakutat Bay, we know that the period of stagnation following the advance began not less than half a century ago, for trees of that age have grown on the stagnant outer portions of these bulbs. A similar statement applies to the piedmont bulbs of Allen, Heney and Miles Glaciers in the lower Copper River valley, though they may possibly be a little younger. Malaspina Glacier maintains some activity throughout most of its area, being motionless only in a few parts where the spreading has been greatest.
Following the expansion of the piedmont bulbs came a cessation of supply sufficient to maintain the bulb portion, and normally one might expect rapid destruction by ablation. In these cases, however, such destruction has been prevented by the concentration of moraine on the surface of the bulbs so protecting the ice as to greatly reduce the rate of ablation. In fact, in some of the outer portions of the bulbs the1 moraine has become so thick that ablation has almost ceased. From the forest-covered bulbs of the Lucia and Atrevida Glaciers, for instance, only trickling streams of water ordinarily emerge, and the stability of the buried ice is so great that dense, continuous forest growth covers the ice. Also, where alluvial deposits have been laid down on the ice, as in the bulb of the Galiano Glacier, the rate of melting of the buried ice must be exceedingly slow. In such cases it is' probable that scores of years, and perhaps even a century or two, will be required to remove the ice from beneath its protective covering of moraine and alluvial deposit. The exceedingly slow rate at which the moraine-covered ice melts, and the fact that Malaspina Glacier inside the moraine-covered portion is not greatly lowered, is interpreted as evidence that this glacier is still in motion throughout most of its area; it promises a rich field for detailed study and one that will probably throw much light on the movement of ice in piedmont glacier bulbs.
Ablation Moraines. In their present wasted stage the piedmont bulbs present several interesting phenomena due to the progress of wastage. One of them is the presence of extensive sheets of moraine, some of which extend up the valley glacier portion. Similar sheets occur on small glaciers which are entirely confined to mountain valleys. Although differing greatly in detail of form, extent, and composition from place to place, the ablation moraines throughout the region have the same general characteristics. Some of the details peculiar to individual glaciers have been considered in the chapters discussing these glaciers; it now remains to consider the ablation moraine in general.
On any individual glacier the moraine sheet is found to vary greatly in thickness from place to place. The broadest areas of thickest moraine are the peripheral portions where stagnation has lasted longest and ablation has, therefore, been most effective. Here